This invention relates to an improved process for the preparation of high molecular weight poly(arylcarbonate)s. The present invention more particularly relates to an improved process for the preparation of high molecular weight poly (arylcarbonate)s wherein the molecular weight is in the range of 45,000-60,000 (expressed as n.sub.inh =0.8 to 1.0) and is increased in a post polymerisation reaction in the solid state. Poly(arylcarbonates)s of high molecular weight produced by the process of the present invention also show enhanced crystallinity.
These poly(arylcarbonate)s are the commercial engineering plastics produced by the industry for replacing glass and metals. Poly(arylcarbonate)s is processed by molding extrusion and film forming techniques for its conversion to different types of useful products such as safety helmets, motorcycle shields, bulletproof windows for cars and unbreakable baby bottles.
Poly(arylcarbonate)s have hitherto been produced by reacting bisphenol-A and phosgene, at an interface of methylene chloride/aqueous sodium hydroxide in presence of either amine or its salt as catalysts. The polymerisation is a two step process involving phosgenation of bisphenol A to its monochloroformate and the condensation of the latter with bisphenol-A hydroxyl groups for producing a polymer that remains dissolved in the organic phase. The overall stoichiometry may be represented as follows:
Amines or its salts act as catalyst for polycondensation. The polymer molecular weight is regulated by termination of the chain with the addition of monofunctional hydroxyl compounds such as phenol or t-butyl phenol. Different ratios of chain stoppers permit the preparation of different grades of polycarbonate corresponding to various molecular weight. However the known toxicity of phosgene and the problem associated with the formation of by-product sodium chloride from this reaction necessitated development of an improved process for the preparation of poly(arylcarbonate)s without the use of phosgene. These processes involve the carbonate interchange reaction of diarylcarbonate or dialkyl carbonate with Bisphenol-A or Bisphenol-A diacetate at elevated temperature 300.degree.-320.degree. C. in presence of suitable catalysts such as alkali and alkaline earth metals, organometallics etc. The molecular weight of poly(arylcarbonate)s is regulated by removal of byproduct (phenol from the former or methylacetate from the latter) which is facilitated by heat and vacuum reatment. Such processes have been discussed in British Patent No. 110,376 and U.S. Pat. No. 4,452,968.
The major disadvantage of the carbonate interchange process mentioned above is that interchange occurs in the melt resulting in the building up of high viscosity of the melt (approximately 6,000-10,000 poise at 250.degree. C.). Removal of the volatile byproduct from such high viscosity of the melt, which is very essential for forming high molecular weight Poly(arylcarbonate)s is very difficult using conventional methods of reactor agitation. This limits the molecular weight produced by melt polycondensation process, to approximately 25,000 to 30,000 (expressed as n.sub.inh =0.4 to 0.5). Such products though useful for moulding applications are unsuitable for extrusion applications which require higher molecular weights, in the range of 45,000-60,000 (expressed as n.sub.inh =0.8 to 1.0).
The main object of the present invention is therefore to provide an improved process for the preparation of poly(arylcarbonate)s of substantially higher molecular weights ranging from 45,000-60,000 (expressed as n.sub.inh =0.8 to 1.0) overcoming the drawbacks of the hitherto known processes. Accordingly, the present invention provides an improved process for the preparation of a high molecular weight poly(arylcarbonate)s, the molecular weight ranging from 45,000-60,000 (corresponding to n.sub.inh =0.8-1.0) using a process employing controlled and programmed heating in the solid state in presence of suitable catalysts.
However, with respect to substantially amorphous aromatic polycarbonates derived from bisphenol-A, having a glass transition temperature (Tg) of 149.degree.-150.degree. C. it has been generally believed that solid state polymerization is infeasible. Such solid state polymerization techniques have been disclosed in the prior art for substantially more crystalline polymers, such as, poly(ethyleneterephthalate), nylon-66, etc. Aromatic copolyester carbonates, containing a crystallizable comonomer such as p-hydroxybenzoic acid (&gt;50%) (Jap. Pat. 55-98224, U.S. Pat. No. 4,107,143) or hydroquinone (Jap. Pat. 52-109591) can also be subjected to the process of solid state polycondensation to increase molecular weights.
In the case of amorphous polycarbonates, derived only from bisphenol-A, prior art reports complex methods of inducing crystallization in prepolymers which can be subjected to solid state polymerization. For example, in PCT int. Appln. No. WO 90/07536, a process is described wherein an amorphous aromatic polycarbonate is treated with a solvent under sufficient shearing force to crystallize the prepolymer. Accordingly, an amorphous polycarbonate prepolymer having a number average molecular weight of about 4,000 is melted and extruded in a strand form at 240.degree. C. through a die having orifices into a bath filled with acetone maintained at 40.degree.-50.degree. C. The bath is agitated at 1000 RPM and the extruded strand is drawn and stretched into a fibre. It is then subjected to shearing action to convert the strands of fibre to particles. The prepolymer, thus treated, is dried and subjected to heating at 220.degree. C. under a flow of nitrogen whereby the number average molecular weight increases to 13,000.
On the contrary, the above referred patent application specifically states that an amorphous prepolymer dissolved in methylene chloride followed by solvent evaporation and drying does not show the desired increase in molecular weight upon solid state polymerization.
The above referred process is beset with the following drawbacks:
1. The process involves an additional step of crystallizing the polymer by extruding and precipitating the molten amorphous polymer into a bath of non solvent for the polymer such as acetone.
2. The process requires heating the polycarbonate prepolymer to a temperature of 240.degree. C. Such thermal processing of unstabilized polycarbonates can lead to unwanted polymer degradation.
3. The hot molten polymer at a temperature of 240.degree. C. is contacted with a low boiling non-solvent at 50.degree. C. This will lead to substantial volatilization of acetone which needs to be condensed for recycling/or disposal.
4. The prepolymer has to be subjected to extensive drying to remove volatile acetone.
5. Acetone is a low b.p./flash point organic solvent and operations described in 1-4 involve hazards of fire and explosion.
The drawbacks make the process described in Int. Pat. Appln. WO 90/07536 difficult to practice and require complex operations involving mechanical and rotating equipments.